The present invention relates to an ink-jet recording apparatus ejecting ink by means of a piezoelectric element, and in more particular to an ink-jet recording apparatus using such an ink whose particles are dispersed in a solvent.
As recording systems having now been practiced, there are a laser recording electrophotographic system, an LED recording electrophotographic system, an impact dot matrix recording system, a thermosensitive recording paper system, a silver halide film recording system, a thermal wax recording system, a thermal dye transfer system, and an ink-jet recording system.
Among them, the ink-jet recording system requires neither a transfer of toner nor a developing process as the electrophotographic system, and this is possible to be miniaturized in a recording system because a recording head has a small size, and to be less in noises because of being not an impact type, and this system has broadly been practiced since copying is possible on plain papers.
Prior ink-jet recording systems include a system using a piezoelectric element deformed in response to electric signals, another using a heating resistor heating in response to electric signals and causing an ink to fly via pressure of boiling by heating, and a further system using electrostatic force by electric signals. The ink-jet recording system using electrostatic force by electric signals is involved with a problem described in Report Magazine of Electronic Communication Associate, ""83/1 vol. J66-C, No. 1, p. 51 that if a distance between recording electrodes is narrow owing to interference of electric fields between adjacent channels, a displacing amount in a position of a pixel is made large.
Therefore, a method, which in spite of a distance between the recording electrodes being narrow, the displacing amount in the position of the recording pixel is not made large, has been developed (JP-W-10-501490) (the term xe2x80x9cJP-Wxe2x80x9d as used herein means an xe2x80x9ca published Japanese national stage of international applicationxe2x80x9d). This is such a method of separating ink droplets selected from an ink main body within the printing apparatus by electrostatic induction force. This method supplies a certain electric potential to a printing head and supplies another electric potential to a platen located in opposition to the recording medium so as to generate an electric field, and since it is unnecessary to modulate the electric field and turn ON per each of exhausted ink droplets, the method does not require any instrument switching high voltage but may use a simple high voltage source for generating electric fields. Further, it is not necessary to separate an electric field to be added to a nozzle from an electric field to be added to an adjacent nozzle, and therefore a distance between nozzles can be made narrow. With respect to a magnitude of the electric field to be given, when an electric field is determined to be a magnitude insufficient to separate an ink droplet, at a time when an ink within the nozzle is at a still position, from the printing head, and when the ink within the nozzle is under a condition of projecting an ink meniscus from a front face of the printing head, the electric field has a sufficient magnitude to separate said ink droplet from the printing head.
This ink droplet selecting method projects the selected ink meniscus from the front face of the printing head. Since a radius of the ink meniscus is small and nearest to an opposite electrode, the projected ink meniscus is accumulated with electric charge, and the accumulated charge is concentrated to an ink having selected a force generated by an electric field. This force overcomes a surface tension of an ink together with pressure of an ink, and separates the selected ink droplets from the ink main body.
The ink droplet selecting means includes {circle around (1)} decreasing the ink surface tension effected with pressure by heat, {circle around (2)} increasing the heat bubbles volume insufficient to cause ink droplets eject, {circle around (3)} piezo-electricity having alternation in volume insufficient to cause eject ink droplets, and {circle around (4)} electrostatic extraction using one electrode per each of nozzles. In addition, the ink droplet separating means includes {circle around (1)} proximity (the recording medium proximate the printing head), {circle around (2)} proximity by pressure of vibrating ink, {circle around (3)} electrostatic extraction, and {circle around (4)} magnetic extraction.
Including the ink-jet recording system of the electrostatic type enabling to reduce the displacing amount, all ink-jet recording systems hold such a problem that ink viscosity increases or an ink is hardened by vaporization of an ink solvent at a nozzle so that picture drawings are made poor.
So, the ink-jet recording apparatus has an instrument for sealing the nozzle at rest of printing, and another one for clearing the nozzle as requested. For such a problem, JP-A-11-192732 (the term xe2x80x9cJP-Axe2x80x9d as used herein means an xe2x80x9cunexamined published Japanese patent applicationxe2x80x9d) discloses a method of ejecting an ink periodically for preventing the recording head from an ink clogging, irrespective of drawing pictures. Further, JP-A-2000-127417 discloses a technology concerned with a cleaning method of the nozzle. The ink to be used to the ink-jet recording system much contains, owing to problems as above mentioned, a water based solvent or an organic solvent containing dye or pigment as a coloring material for lowering the ink viscosity and particle concentration.
In these ink-jet recording systems, thereby, blur appears in drawn images, and it is difficult to form images of high quality and high resolution, and it takes a long time for drying the image forming parts, so that it is difficult to heighten productivity.
On the other hand, for accomplishing an ink-jet recording system being less blur in images, fast at drying speed and of high quality and high productivity, an ink, particles of which are dispersed in a solvent is disclosed in Japanese Patent 3,000,672 and JP-A-2000-63723. For lessening blur of ink images and accelerating a drying property, it is effective to increase the concentration of particles and decrease the concentration of solvent.
However, if applying such an ink of increasing the concentration of particle and decreasing the concentration of solvent to conventional ink-jet recordings, an ink clogs at a nozzle to hinder a good image drawing.
For settling this problem, JP-A-8-142331 describes an ink-jet recording apparatus being less blur in images, fast at drying speed and of high quality and high productivity. This is composed to use an ink of coloring materials electrically charged to a predetermined polarity and dispersed in a solvent, and regulate flowing of the coloring material component in an ink by force of an electric field so as to condense the coloring material component, and an ejecting instrument ejects the condensed coloring material component to the recording medium by the force of electric field.
This apparatus is further provided with a removing instrument in which, being based on a condensing polarity at a downstream in an ink sending direction of a pair of condensing electrodes, voltage of the same polarity as a charged polarity of the coloring material is applied to a condensing electrode at an upstream of the ink sending direction, so that the coloring material component remaining between said pair of condensing polarities is sent to the downstream in the ink sending direction for removing the remaining coloring material component.
There is further provided a recovering device for recovering the ink, the coloring material component of which is condensed by a condensing instrument, through a recovering inlet located nearly the condensing electrodes.
But, this ink-jet recording apparatus has a fatal defect that since this ink-jet recording apparatus ejects an ink by the electric field, a strong electric field is necessary, so that if making the strong electric field, since interference of the electric fields occurs accordingly between adjacent electrodes, the adjacent electrodes must be separated at some degree, thus the channel density cannot be made large.
A further defect is that when ejecting the coloring material component condensed by the condensing means, since the whole of the condensed coloring material component is ejected by the condensing means, a concentration does not go up as desired depending on conditions.
The removing instrument of the residual coloring material component is for removing the coloring material component remaining between the condensing electrodes, but cleaning thereby is done only at the inside of the nozzle, and the residual coloring material component at a front end of the nozzle contacting an air cannot be removed.
This invention has been established, noticing the above mentioned problems, and it is accordingly an object of the invention to offer an ink-jet recording apparatus ejecting an ink by use of a piezoelectric element which is less blur in images, fast at drying speed and of high quality, enabling to make an image resolution large and high productivity.
For accomplishing the object, an invention of the ink-jet recording apparatus described in a first aspect of the invention is an image forming apparatus using an ink, electrically charged particles of which are dispersed in a solvent for ejecting ink droplets by means of a piezoelectric jet typed ink jetting head (called as xe2x80x9celectric field jetting headxe2x80x9d hereafter), characterized by providing ink-sending means for sending an ink to the jetting head; particle distribution-controlling means for controlling distribution of particles concentration in said ink within the jetting head; image-receiving sheet supporting-transporting means for supporting and transporting the image-receiving sheet for receiving flying ink droplets ejected from the jetting head; and accelerating electric field-forming means for forming an electric field accelerating speed of the flying ink droplets.
Another invention described in a second aspect of the invention as set forth in the first aspect is characterized by providing ink recovering means for returning an ink from the jetting head.
A further invention described in a third aspect of the invention as set forth in the first or second aspect is characterized by ejecting substantially only a solvent carrier by use of the particle distribution-controlling means after finishing an image recording or with a cleaning mode.
A still further invention described in a fourth aspect of the invention as set forth in the first aspect is characterized by providing the nozzle of the jetting head only in the vicinity of a densest location in a coloring material concentration distribution formed by the particle distribution controlling means.
Namely, according to the invention, since an ink is ejected by the piezoelectric element, no interference of the electric field with an adjacent channel exists and the nozzle can be formed at high density.
Further, by accelerating speed of the flying ink droplets by the accelerating electrode, a distance between the jetting head and the image-receiving sheet can be broadened, enabling to improve a landing precision of the ink droplet.
The particle concentration in the ink droplet to be ejected is heightened by heightening, at recording images, the particle concentration near the nozzle by the means controlling the particle distribution within the jetting head, and after recording images the particle concentration near the nozzle is decreased, and the nozzle can be cleaned by ejecting substantially only the solvent. As a result, it is possible to offer the ink-jet recording apparatus being less blur in images, fast at drying speed and of high quality, and high productivity.